Fiber optic telecommunications lines are proliferating because of their enhanced reliability in comparison to electrical circuits and their greatly enhanced bandwidth. Electro-optical telecommunications interfaces have therefore been developed to translate local electrical telecommunications traffic to optical telecommunications traffic of much higher frequencies, and vice versa.
Conventional optical telecommunications systems have their optical fibers arranged in pairs. In a so-called "one for one" operation mode, a pair of "A" fibers are conventionally designated to carry the telecommunications voice and data traffic, and a pair of "B" fibers are designated as the protect/channel access fibers. If there is a failure in the "A" fibers, the voice and data traffic is switched to the "B" fibers.
In most conventional fiber optic telecommunications terminals, each of the fibers is associated with an STS formatter or deformatter. "STS" is a digital electronic telecommunications protocol operating at about 51 MHz. An STS formatter converts a received STS signal into a formatted electronic signal that is ready to be converted into an optical signal. So-called "transmit" optical fibers are associated with such formatters. A deformatter takes a formatted signal resulting from a received optical signal and turns it into an STS signal. So-called "receive" optical fibers are associated with deformatters. Conventional terminals have an "A" formatter, a "B" formatter, an "A" deformatter and a "B" deformatter.
Conventional electrooptical terminals typically also have two sets of DS3/STS interface cards. "DS3" is an analog telecommunications protocol operating at about 41 MHz. The first set of cards is connected to all four formatters and deformatters, such that STS signals may be sent to and received from either the "A" formatter and deformatter, or alternatively the "B" formatter and deformatter. A second set of interface cards is connected to only the "B" formatter and deformatter.
The STS data are "stuffed" into a clock frame and then are transmitted to and from the formatters and deformatters. The clock frame in turn depends on a clock signal. This clock signal conventionally has either of two origins. In one prior art scheme, the "A" formatter has a "master" clock to which a clock generator on the "B" formatter is slaved. STS data are received from the interface cards by the "A" formatter as using an "A" clock frame, while STS data received from the interface cards by the "B" formatter uses a "B" clock frame. This scheme has a drawback in that if the "A" clock generator fails, the "B" clock will also fail temporarily, causing a hit on both "A" and "B" traffic. The problem has been obviated in the past with the employment of expensive "holdover" circuitry in conjunction with the "B" clock generator.
In another prior art scheme, clock generation circuitry is placed on a separate card, which is then used as a central source for all clocking functions. However, a separate clock card is undesirable due to costs associated with having yet another separate component of the terminal. These additional costs are attributable, for example, to increased installation, maintenance and inventory costs. A need therefore exists for clock circuitry which is adapted to be used in electrooptical terminals with both "A" and "B" fibers, but which avoid the problems associated with the above prior art clocking schemes.
Conventionally, each of the DS3/STS interface cards is capable of receiving only one of the "A" STS signal, originating from the "A" deformatter, and the "B" STS signals, originating from the "B" deformatter. A conventional DS3/STS interface card does not have the ability to inspect the quality of that STS signal that it is not currently receiving. In order to compare the quality of the two STS signals, it must look at the current signal, switch to the other signal, look at the other signal, and make a comparison between the "A" and the "B" signal. If it turns out that the "B" signal is worse than the "A" signal, it must switch back again. Every switching event causes a hit to the telecommunications traffic. As they switch between "A" and "B" paths to inspect signal quality, conventional DS3/STS interface cards cause many hits to the traffic that turn out to be unnecessary. A need therefore exists to develop an interface card which will protect the traffic with fewer switching events.
Electrooptical terminals may be used to form "add/drop multiplexer" nodes, at which DS3 traffic is "added" to a stream of optical traffic and at which some of the optical traffic is "dropped" to the DS3 protocol. These nodes also pass through optical traffic to other electrooptical nodes. Terminals are also commonly configured as portions of unidirectional and bidirectional rings. As so configured, these nodes have both add/drop and passthrough capabilities. In order to "pass through" optical traffic, conventional nodes must translate the optical traffic to STS format, must translate the STS format to DS3 format, must transfer the DS3 signals to other DS3 cards, must translate the DS3 signals to STS signals, and must translate these STS signals to optical signals for retransmission to other nodes. This circuitous retranslation path requires additional DS3/STS interface capability, increases equipment complexity, and provides further opportunities for signal degradation and equipment failure. A need therefore exists for an electrooptical telecommunications node at which the optical/STS/DS3/DS3/STS/optical translation path is not necessary for through-transmission of optical signals.
Conventional formatters receive a plurality of channels of STS information from DS3/STS interface cards, such as 12. Only certain of these channels may be carrying voice or data information at any one time. For example, in a terminal where there are three STS channels transmitted from each of four DS3/STS interface cards, there may be information transmitted on only channels 1, 5, and 10. Conventional formatters and associated optical transmitters merely translate these channels and the remaining 9 empty channels into corresponding optical channels, leaving 9 unused channels interspersed among the used channels. In this example, the channels would merely be translated back into STS at the next node, and possibly routed to DS3/STS interface cards which correspond to the interface cards from which the channels emanated. This results in requiring more multichannel DS3/STS interface cards than would otherwise be needed. A need therefore exists to more efficiently use the optical bandwidth and to avoid the provision of further interface cards to allow for nonuse of some of the interface card receiving channels.